The present invention relates to a process for manufacturing a magnetic powder used for manufacture of a magnetic recording medium.
Recent magnetic recording media such as magnetic recording tapes adopt the vertical magnetic recording method which utilizes magnetization in the direction perpendicular to the surface of the magnetic recording medium in order to improve the recording density. The vertical magnetic recording method is suitable for high density recording since the magnetic field is reduced more as the recording density increases. The magnetic recording medium according to this recording method requires a magnetic recording film which has an axis of easy magnetization in the direction perpendicular to the surface thereof. Such a recording film is formed by sputtering Co-Cr or by coating magnetic fine grains of hexagonal ferrite on a base material.
The formation of the magnetic recording film by coating will be described in more detail. The magnetic fine grains are mixed with a suitable binder and the mixture is coated on base material such as a nonmagnetic tape. Subsequently, by arranging the coating in a magnetic field such that the surface crosses perpendicularly the direction of the magnetic field, the axis of easy magnetization of the respective fine particles in the coating are aligned along the direction of the magnetic field. Finally, the coating is dried to provide a recording medium with a recording film for vertical magnetic recording.
In the manufacture of such a magnetic recording medium, several factors are important in relation to the characteristics of the obtained magnetic recording medium. First, hexagonal ferrite has a very high coercive force, resulting in a defect in that the magnetic head becomes saturated during recording. For this reason, the coercive force must be reduced to a value suitable for vertical magnetic recording, for example, about 1,000 Oe, by substituting particular atoms with part of the atoms of the ferrite. Secondly, the grain size of the crystalline fine grains of the hexagonal ferrite must be selected to be within the range of 0.01 to 0.3 .mu.m. If the grain size is less than 0.1 .mu.m, a magnetic effect strong enough for magnetic recording may not be obtained. When it is over 0.3 .mu.m, a multi-magnetic domain results. Therefore, the coercive force obtained becomes smaller than the desired coercive force. And the surface coarseness of the magnetic recording film becomes great. These are disadvantageous for vertical magnetic recording. Thirdly, the magnetic fine grains having the grain size as defined above must be uniformly dispersed within the binder. The magnetic fine grains tend to flocculate due to their own magnetism and the flocculation force is strong. When magnetic fine grains thus flocculated are used, the surface precision of the magnetic recording medium obtained is not good. This adversely affects various characteristics causing noise generation and a drop in output.